1. Introduction

Real-time physical, chemical and biological sensing are critical in many applications, such as environmental pollution measurement, health care, gas detection and industrial processes control. Major requirements are measurements at multiple locations, large spatial area coverage, operation capability in harsh environments and low cost. Particularly, there is a critical need for technologies that can provide distributed or quasi-distributed measurement at multiple points. Fiber optics sensors are ideal transducers for applications requiring devices that are durable, stable and insensitive to external perturbations [1].

The challenging goal of this work was to find an all-in-fiber coupling method to implement a quasi-distributed interrogation of whispering gallery mode (WGM) micro-optical resonators. In fact, high quality factor (Q) WGM micro-resonators are known to exhibit unique properties for sensing by using different geometries [2]. Light can be stored for a long time in small volumes thus increasing tremendously the probability to interact with the external environment during the numerous roundtrips at the resonator surface. Any small change in size and/or refractive index at the cavity surface induces, through the interaction with the evanescent part of the WGM field, either a change in the resonator Q-factor or a shift in the resonance frequency [3]. As an example, highly sensitive biosensors able to detect single nanoparticles or molecules have been demonstrated [4]. However, in case of multiple sensing a critical point for this type of micro-optical transducers is the development of a coupling system able to interrogate a series of cavities along the same fiber, in order to implement a quasi-distributed sensor. So far, none of the several methods [5] that have been developed to efficiently couple light to WGM micro-resonators simply fulfils this key request. With one or more standard fiber tapers [6], for instance, it is impossible to selectively excite micro-resonators and it is extremely critical to distinguish (a priori) the resonances of each cavity by collecting the signal at the fiber end. Additionally, standard fiber tapers used for coupling are very thin, down to 1 μm in diameter, and therefore they are very fragile.

In previous works [7,8], we proposed a configuration for coupling light to a high-Q silica WGM resonator, which is based on a long period grating (LPG) written in silica fiber followed by a thick fiber taper. The LPG allowed wavelength selective excitation of high-order cladding modes, so that thicker and more robust taper (with waist diameters in excess of 15 μm, which are easy to fabricate and do not degrade in time) could be used for coupling light from the fiber to the WGM resonator. This configuration is more robust than the standard fiber taper coupler but it does not allow interrogating more spheres coupled to the same fiber by monitoring the transmitted light. The only possibility of multiplexing consists of collecting the scattered light from each resonator and eventually sent the signal to a photodetector [8], which is a quite complicated approach. Conversely, the transmitted light cannot be used for multiplexing. In fact, in a first configuration, if the fiber jacket is kept after the taper, it cuts off the modulated cladding mode, whereas, in a second case, if the jacket is removed, then the cladding mode excited by the LPG would be coupled to all the resonators following the LPG.

Herein, to overcome these limitations, a new all-in-fiber coupling system for quasi-distributed and wavelength selective addressing of several WGM resonators is proposed. This approach consists of replicating many times the same structure that is based on a pair of identical LPGs with a fiber taper in between. Compared to the previous approaches [6,7], the presence of the second LPG allows coupling of the light back into the core and therefore all the necessary information is contained into the core mode, which is transmitted up to the end of the fiber and collected by a single photodetector. By fabricating pairs of identical LPGs operating in different wavelength bands, it is possible to achieve multiple selective coupling and interrogation (or selective addressing) of spatially quasi-distributed micro-resonators by using the same optical link.

2. Fabrication and experimental setup

A schematic representation of the coupling mechanism to WGM resonators by using the proposed approach is depicted in Fig. 1. First, adiabatic fiber tapers were fabricated by a heating and pulling procedure [9] in a single-mode boron-germanium co-doped optical fibers (Fibercore PS1250/1500) with core and cladding diameters of 6.9 μm and 124.6 μm, respectively. Tapers with a diameter of 15–18 μm were fabricated allowing the transmission of the selected cladding mode. For efficient coupling of these cladding modes to silica WGM resonators, partial tapering of the fiber is necessary in order to reduce the optical field size and increase its external evanescent portion [7]. However, as already pointed out, the tapered region is one order of magnitude thicker than that of the standard tapers (1–2 μm), and therefore it is much more robust for practical applications.

 

Fig. 1 Schematic representation of the fiber based coupling unit consisting of a pair of LPGs and a taper in between.

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After the taper fabrication, the coupling unit was completed by fabricating the pair of identical LPGs on both sides of the tapered fiber (see Fig. 1). An LPG is characterized by a series of periodic refractive index changes in the core of a single-mode optical fiber, with a grating period Λ ranging from 100 μm to 600 μm. In such structure, the coupling occurs between the fundamental core mode (LP₀₁) and co-propagating azimuthally symmetric cladding modes (LP_0m, m ≥ 2), each of which generates an attenuation dip in the fiber transmission spectrum [10]. The gratings were inscribed with a point-by-point technique by using a KrF excimer laser (Lambda Physic COMPex 110) [11]. The first LPG selectively couples the light from the core mode to a specific cladding mode, which evanescently excites the resonator WGMs in the tapered section. The resonator is placed in contact with the taper for improved stability. The second LPG couples the light back into the fiber core, thus all the information is contained in the core. This structure can be replicated many times on the same fiber allowing wavelength selective addressing of more resonators. Two different pairs of LPGs with Λ of 340 μm and a length L_LPG of 17 mm (for the first coupling unit) and with Λ of 365 μm and a L_LPG of 21.9 mm (for the second coupling unit) were fabricated.

For proving the effectiveness and feasibility of the proposed system, two types of resonators, silica microspheres [9] and microbubbles [12] were fabricated in our labs. Diameters ranging from 260 μm to 290 μm for microspheres and from 380 μm to 500 μm for microbubbles were selected. In both cases, the resonator size was large enough so that the free spectral range (FSR) was significantly smaller than the LPGs bandwidth [7].

The experimental setup used for the characterization of the transmission spectrum consists of two fiber pigtailed tunable external cavity lasers (Anritsu Tunics Plus, linewidth 300 KHz), covering the bandwidth from 1390 nm to 1640 nm. An optical spectrum analyzer (OSA–Ando AQ6317B) detects the signal. The coupling approach was tested by means of the same laser sources − that can be finely and continuously swept in wavelength within few GHz − and a single photodetector connected to an oscilloscope. The experiments were performed in air but, because of the improved taper robustness, we plan to design a similar coupling unit to operate in an aqueous environment.

3. Results and discussion

The spectra of the two single LPGs (before making the second identical ones to form the two pairs) were first recorded and are shown in Fig. 2 merged together (red line). The two attenuation bands at wavelengths of 1518.99 nm and 1613.31 nm represent the coupling with the same LP₀₇ cladding mode achieved with different Λ, 340 μm and 365 μm, respectively. The band depth exceeds 15 dB for both LPGs because high coupling efficiencies with the cladding modes were targeted. Figure 2 also details the spectrum measured by putting in series the two coupling units, i.e. the two pairs of LPGs with the respective tapers (blue line). Typical interference patterns can be observed as, for each LPG pair, the system acts as a Mach-Zehnder interferometer (MZI) [13]. The maximum modulation amplitudes (best MZI contrast) corresponding to the 3 dB attenuation wavelengths of the single LPGs (see the dashed grey line in Fig. 2) appear as two fringe patterned sidebands symmetrically placed on both sides of the single LPG minimum transmission wavelengths (1518.99 nm and 1613.31 nm). At these wavelengths, instead, a reduced modulation amplitude and a rather high transmission can be observed, showing that the light is effectively coupled back into the core mode by the second LPG after the tapered sections. Minimum insertion losses are about 1 dB at 1518.99 nm and about 2 dB at 1613.31 nm. In addition, Fig. 2 shows that the period of the interference fringes related to the second coupling unit (Λ = 365 μm) is smaller than that of the first one (Λ = 340 μm). This can be ascribed to the greater physical distance L_tot (see Fig. 1) of the two LPGs composing the second coupling unit (roughly 5.2 cm) with respect to the first one (roughly 4.4 cm). As a number of external perturbations, such as fiber bending, temperature and refractive index, affects the transmission spectra of the coupling unit, an initial calibration of the system acquiring these spectra (in order to assess the operating central wavelength) will be needed for optimal performances.

 

Fig. 2 Transmission spectra of the two single LPGs (solid red line) showing minimum transmission at the wavelengths of 1518.99 nm (Λ = 340 μm) and 1613.31 nm (Λ = 365 μm), respectively, and of the two LPG pairs together (solid blue line). The grey line crosses the −3 dB value of the attenuation dips of the single LPG and corresponds to the maximum LPG-based MZI contrast.

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Each coupling unit was individually tested and high-Q WGM resonances in both microspheres and microbubbles were effectively excited. The resonators were placed in contact with the tapers. Phase-matching conditions between the LPG cladding modes and the WGMs were fulfilled thanks to the radial and azimuthal high-order modes of spherical WGM resonators [8]. The transmission dips were fitted by a Lorentzian function obtaining typical Q-factor values close to 10⁸ for both types of micro-resonators. Figure 3 shows the WGM resonances excited both in microspheres (Fig. 3(a)) and microbubbles (Fig. 3(b)) together with a 3D sketch of the coupling system for both microcavities. The main graphs show the maximum Q-factors achieved for each type of resonators, while the insets show the maximum contrast achieved with the corresponding microcavity. By optimizing the state of polarization of the input light (coupling units are polarization insensitive [7] but the WGM distribution in the resonator depends on the polarization) and the resonator position along the taper, we observed maximum coupling efficiency (or resonance contrast) of about 50%–60%. In our case, the system acts as a multimode waveguide coupler (not all the lunched power is in the selected cladding mode) and thus the contrast is limited, since it is not possible to reach the condition of single-mode coupler [14]. The coupling efficiency has been significantly improved as compared to our previous work [7]. The reason could be related to the fact that the cladding mode is coupled back in the core right after interacting with the resonator, and once in the core there are no additional losses. In the previous configuration, the cladding mode was left propagating at the fiber-air interface along the whole remaining part of the fiber, possibly introducing additional losses in the modulated signal (and thus reducing the resonance contrast). A wavelength range up to about 15 nm for each coupling unit (from 1510 nm to 1525 nm for the first LPG pair and from 1605 nm to 1620 nm for the second one) was considered for efficient coupling, as within this range up to 90% of the power is coupled from the fundamental mode to the LP₀₇ cladding mode (see single LPG transmission spectra in Fig. 2). In order to confirm that the light can be selectively coupled in the micro-cavities, the laser source was also tuned outside the attenuation band of the LPGs, for instance from 1545 nm to 1585 nm (see Fig. 2), and no significant coupling was observed as expected.

 

Fig. 3 Sketch of the LPG pair-based coupling units exciting WGMs in both microspheres (a) and microbubbles (b), along with an example of typical WGM resonances obtained for both micro-cavities. The two insets show the maximum coupling efficiency obtained for both types of micro-cavities.

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After this individual characterization, the system composed by the two in-series coupling units was analyzed. Figure 4(a) show the resonances obtained by scanning (about 2 GHz) around the LPG central wavelengths (1518.99 nm for the first coupling unit and 1613.31 nm for the second one), when both resonators were in contact with their respective tapers (as sketched on the top of Fig. 4(a)). Typical Q-factor values comparable to those previously mentioned were observed. The selectivity test was carried out by alternatively de-coupling one of the resonator and by looking at the transmission spectrum of the other micro-cavity, as shown in Fig. 4(b) (first resonator in contact, second resonator detached, as sketched on the top of the figure) and in Fig. 4(c) (second resonator in contact, first resonator detached). In particular, several measurements were performed varying the taper coupling position along the azimuthal axis of the resonators. It was found that each resonator was independently excited and the resonances of one resonator were not present in the ‘wavelength range’ of the other one. Therefore, the proposed coupling system can be used for quasi-distributed sensing. In our case, considering a wavelength range of 50 nm ‘reserved’ for each LPG pair (a range larger than the 3 dB bandwidth for each LPG can be considered in order to safely avoid any cross-talk) and the source bandwidth of 250 nm, up to 5 coupling units could be fabricated along the same fiber, leading to possible multiplexing of up to 5 different micro-cavities selectively coupled and independently interrogated. Considering that typical insertion losses of the single coupling unit are 2 dB, total losses not exceeding 10 dB for a set of 5 units can be assumed.

 

Fig. 4 Sketch of two in series coupling units with both resonators (circle) coupled to each tapered section of the fiber and corresponding resonances obtained by scanning the laser source around the LPGs central wavelengths (0 MHz detuning) (a). First resonator in contact, second not (b). Second resonator in contact, first not (c). The resonances of each coupling unit remain unchanged proving they are independently excited without cross-talk.

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4. Conclusions

An independent and selective coupling with high-Q WGM micro-resonators was demonstrated based on a new all-in-fiber method consisting of a pair of long period gratings with a fiber taper in between. The first LPG is used for the selective excitation of cladding modes, whereas the taper allows evanescent coupling from the fiber to the resonator WGM. The second LPG couples the light modulated by the resonator back into the core. The proposed structure is robust and easy handled thanks to the use of a thick taper (15–18 μm waist), and allows collecting light just from the fiber core thanks to the second LPG. The structure can be replicated many times along the same fiber depending on the source bandwidth and all the information can be independently collected at the end of the fiber by using a single detector. Q-factor of the order of 10⁸ and total coupling efficiency up to 60% were experimentally achieved by using silica microspheres and microbubbles. To the best of our knowledge, this approach allows for the first time a quasi-distributed and wavelength selective addressing of different WGM micro-optical resonators along the same fiber with an almost-zero resonance cross-talk, thus improving the perspective of optical fiber-based sensing and biosensing. In addition, the proposed system results to be a very promising platform for multiplexing hollow WGM microstructures, like microbubble resonators, which are intrinsically suited for integrated microfluidics [15].